The invention relates to an electrochromic cell comprising two paired, transparent plates with a spacing between them,
which are each provided on the side facing the other with an electrically conductive electrode layer which extends over the entire area of the plate and is in each case connected to an external electric connection,
which are joined to one another so as to form a seal by means of a bead of adhesive running round the edge region of the plates and in each case leaving a narrow margin free, and
between which an electrochromic medium is located.
Electrochromic cells, also known as EC cells for short, exploit the reversible change in the colour and/or the optical density in an electrochromic medium, which change is obtained by means of an electrochemical redox reaction which occurs in this electrochromic medium in which the oxidized state and the reduced state have different colours and/or optical densities. Such electrochromic materials alter their optical properties as a result of the action of an electric field; they can be brought back to their initial state by application of a field of opposite polarity.
The EC cells typically comprise two plates which are preferably made of glass, i.e. a front glass and a back glass, which are joined to one another at a spacing and sealed from the surroundings along their periphery. Between the two plates there is the electrochromic, i.e. optically effective, medium, in particular a liquid containing viologens. Each plates is provided on the side facing the EC medium with an electrically conductive electrode layer which covers the entire surface of the plate and is in each case connected to an external electric connection. If a potential is applied to the electric connections of the two planar electrodes, the absorbance of the EC medium located in front of the back glass changes, i.e. the cell loses transparency.
These relationships are prior art and have been disclosed in numerous documents, e.g. U.S. Pat. No. 4,917,477.
Such electrochromic cells are preferably employed in the automobile industry, in particular as automatically dimming interior and exterior mirrors in motor vehicles. In such an application, a mirror layer is located on the back glass of the cell, which is then known as a mirror cell; this mirror layer can also be formed by the associated electrode layer.
Such automatically dimming mirrors increase road safety for night driving by shielding the eyes and help to avoid accidents. For this purpose, the dimmable mirror systems have photoelectric sensors for detecting light which may dazzle the driver; the sensors are installed on the vehicle pointing in the direction from which the light impinges on the mirror. These sensors recognize the danger of dazzling in an instant and dim it to a level more bearable to the eyes by a sliding reduction in the mirror reflection to 10% over a few seconds. When the danger of dazzling has passed, the mirror reflection immediately increases again to the initial value. This automatic darkening and brightening of the EC mirror is repeated every time there is a risk of dazzling throughout the entire life of the vehicle without any reduction in effectiveness.
EC cells can also be employed as screens against the sun and to protect the eyes in motor vehicles and buildings. In such an application, both electrically conductive electrode layers are made transparent and no mirror layer is provided.
A decisive factor in the performance of an EC cell is the spacing of the two plates, which is in the range from about 0.1 to 0.2 mm. This small spacing also causes problems in the joining of the electrically conductive electrode layers on the insides of the plates.
It is known that the spacing between the glass plates can be set by mixing glass spheres having the appropriate diameter as spacers into the adhesive for the peripheral sealing bead of adhesive. This adhesive has to be specially produced and is thus made much more expensive. In addition, glass spheres having different diameters for different spacings have to be made available as spacers, which likewise contributes to a great increase in cost. Furthermore, it has to be ensured during the production of the cell that it is only the glass spheres which determine the spacing, i.e. that there is no adhesive between the surface of the glass sphere and the plate, and also that no leakage paths are formed between adjacent glass spheres.
A further problem in such a typical cell is making the contact between the external connections and the associated planar electrode while maintaining insulation from the other electrode, since the planar electrodes are superposed with coincident edges and separated by only a very narrow gap of from about 0.1 to 0.2 mm.
U.S. Pat. No. 5,151,824 has disclosed solving the problem by front and back glass being offset to one another by a predetermined amount, so that each glass then has a free zone of planar electrode which can be utilized for making the contacts. To each of these marginal zones there is fitted an elongated contact clip with spring finger contacts embracing the glass with the free marginal zones of the planar electrodes, and the connecting wire is in each case soldered to this contact clip.
The edge offset in this known EC cell increases its dimensions, which is not desirable, particularly not when the application is as EC mirror for motor vehicles. The automobile industry wants EC mirrors whose dimensions are virtually no different from the conventional mirrors. In addition, the spring or clip contacts are very expensive and inconvenient to fit and there is a need for soldering procedures which complicate the production process and result in not inconsiderable production costs. Moreover, the contact to the planar electrode is established only in a comparatively narrow region. This has an adverse effect on the speed with which the absorbance of the EC medium changes.
EP 0 434 453 B1 (=U.S. Pat. No. 5,066,112) has disclosed an EC mirror which has no offset of the plates of the optically effective cells and no spring contacts in the form of clips; rather, an additional conductive contact layer is applied to the planar electrode in the marginal zones of the plates including their end face and the connecting wire is then soldered onto this contact layer on the end face. Such an EC mirror is firstly very expensive to produce and, secondly, the end-face contact zone for attaching the connecting wire is very narrow so that it can easily tear off and, furthermore, there is only a small contact area which likewise has an adverse effect on the speed with which the absorbance of the EC medium changes. In addition, complicated and expensive soldering procedures are necessary.
It is an object of the invention, starting from the EC cell described at the outset, to design this such that the spacing between the two cell plates can be predetermined in a simple manner a solder-free connection of the electrode layers to external connections is possible.
This object is achieved according to the invention by a laminate of metal/solid nonconductor/metal layers being located around the periphery in the space between the two margins.
This laminate which can be produced at a constant thickness thus serves, in a simple way, as spacer which ensures a uniform, predetermined cell spacing. The metal layers of the laminate additionally allow the contacts to be made to the electrode layers of the cells and offer the possibility of a solder-free connection technique. The laminate according to the invention is therefore a multifunctional laminate.
The laminate according to the invention therefore makes it possible to dispense with mixing glass spheres into the adhesive, which enables a commercial adhesive to be used in place of an expensive special adhesive containing expensive-to-produce glass spheres. The laminate also offers the simple possibility of varying the spacing of the plates of the cells over a wide range by simple variation of the thickness of the metal layers which are preferably produced from metal foils or of the thickness of the solid nonconductor.
Since the peripheral laminate with the peripheral metal layers prescribes the maximum possible contact length to the electrode layers and thus for application of a potential, a very fast change in the absorbance of the EC medium is achieved.
According to an embodiment of the invention, a predetermined part of the laminate projects as a contact strip beyond the edge of the plates of the cells.
In such an embodiment, the two outer metal layers of the contact strip can be connected simply and without solder to an external connection, e.g. in an external EC mirror of a passenger car which has a heating foil connected via a plug to the electrical system of the car, by adhesive bonding to the heating foil after bending the contact strip over the edge of the cell.
The laminate according to the invention thus eliminates the soldering procedure, which considerably simplifies the production process and reduces the production costs. In addition, no additional parts such as contact clips and wires are required.
According to a further embodiment of the invention, the solid nonconductor of the middle layer is formed by a plastic. Appropriate selection of the plastic enables the laminate to be made flexible but still not deformable in respect of its thickness.
If the cell contains a liquid EC medium which is only introduced into the cell after assembly, the laminate and the bead of adhesive have, according to a further embodiment of the invention, a closable opening at at least one point for filling the cell with the electrochromic medium.